Not Applicable.
1. Field of the Invention
This invention relates to a method and apparatus for prospecting and geophysical surveying using seismoelectric signals. This can be used for a geophysical survey, either at the surface, above the surface, in a borehole, or in the laboratory. One feature of the invention is that the seismic wave source may be artificial or natural. The method of the invention then analyzes the seismoelectric signal utilizing the relationship that the seismoelectric signal""s source travels with seismic wave velocity and frequency and carries information about underground formations. The invention can thus be used for survey of underground formations and also, in particular for of permeability. Alone or when combined with the seismic wave data, seismoelectric signals provide a more detailed and less costly geophysical survey.
2. Description of Related Art
The traditional seismic survey uses geophones, essentially very sensitive geophones placed in contact with the earth""s surface or the walls of boreholes in order to capture seismic signals (vibrations in the medium) which are generated by any of a variety of sources, natural, and artificial. These surveys have a number of disadvantages: complexity, cost, and the fact that the geophones must receive a certain threshold value of seismic signal in order to be activated and record the signal.
Low frequency electric signals have been known to propagate through the Earth""s crust for some time. The sources of these signals are numerous. Geotelluric signals are caused by geological reactions within the Earth. The telluric signal is caused by the solar wind, a steady flow of electrically charged particles emanating from the sun. Seismoelectric signals are caused by the interaction of a geological matrix such as permeable earth material, and water within it, under the effect of a seismic disturbance. Other causes of low frequency subterranean electrical signals are also known.
These various types of signals have been hopefully examined by surveyors for decades, without any useful theoretical basis for scientific analysis of the results.
In the realm of more specific and scientifically verifiable known methods, there are several patents which teach use of streaming potential in measurement of permeability of geological samples.
Streaming potential is the electrical potential (voltage) generated by water flowing in a solid matrix such as permeable rock. The opposite effect, electro-osmosis, is the generation of water flow or water pressure in a permeable matrix by application of an electrical potential across the matrix.
U.S. Pat. No. 3,599,085 teaches use of a sonic transducer periodically exciting a formation (matrix) at low frequencies to cause periodic electrokinetic potentials which are measured at a location near to the transducer and at a location spaced from the transducer, the ratio of the measured potential being related to the electrokinetic skin depth to provide an indication of the permeability of the formation. U.S. Pat. No. 4,427,944 teaches application of pressure of alternating polarity to the matrix and measurement of the generated transient streaming potential in the time domain to estimate the characteristic response time of the matrix. U.S. Pat. No. 5 4,742,402 teaches the building of a seismoelectric signal recording device. Finally, U.S. Pat. No. 5,417,104 and its continuation, U.S. Pat. No. 5,503,001, teach determination of permeability of porous media and thickness of mudcake on the walls of boreholes and thin porous media by measuring at a finite frequency streaming potential induced in such thin layers by finite frequency pressure oscillations. This method uses multi-chambered apparatus, requires that the electrodes all be quite close to the source of the pressure oscillations, and requires measurement of the extremely small phase shift between the components of the streaming potential signal.
These methods also appear to lack data handling methodology based on an understanding of the nature of the propagation of seismoelectric signals, as follows. The streaming potential creates a seismoelectric signal, energized by the passage of a seismic wave. A secondary electromagnetic field is induced by the seismoelectric signal, which can be detected in its proximity. The source of the seismoelectric signal is the passage of the seismic wave, and the seismoelectric signal source thus betrays the location, velocity and frequency of the seismic wave. Since the known methods do not teach that seismoelectric signals are propagated at the velocity and frequency of the seismic wave, they also do not point to convenient, low cost methods for using seismoelectric signals for surveys in a wide variety of environments: boreholes, on the surface, above the surface, or in the lab. Furthermore, seismoelectric signals are related not only to permeability and resistivity, but also to seismic wave forms dependant upon the P-wave (compression) and S-wave (shear) velocities. As a result, the known art methods are limited to special cases such as lab work, limited geological formations or determination of the thickness of mudcake in a borehole.
It is one object of this invention to overcome many of the disadvantages of known streaming potential measurement methods.
It is another object of this invention to provide a method of geophysical surveying which does not require a threshold value of seismic signal in order for the seismic signal to be detectable.
It is another object of this invention to provide a method of use of the relationship between seismic waves and seismoelectric signals in seismoelectric surface prospecting.
It is another object of this invention to provide a method of use of the relationship between seismic waves and seismoelectric signals in streaming potential measurements inside a borehole to evaluate permeability.
It is another object of this invention to provide a method to detect three dimensional seismic wave signals using an aerial antenna instead of geophones.
It is another object of this invention to provide a simple, low cost method for conducting accurate and quantifiable geophysical borehole surveys and geophysical borehole prospecting.
It is another object of this invention to provide a simple, low cost method for conducting accurate and quantifiable geophysical aerial surveys and geophysical aerial prospecting.
It is another object of this invention to provide a simple, low cost method for conducting accurate and quantifiable laboratory testing of geological samples.
It is yet another object of this invention to provide a low cost method for accurate, quantifiable surveying without use of expensive equipment.
It is yet another object of this invention to provide simple, low cost apparatus for conducting accurate and quantifiable geophysical surface surveys and geophysical surface prospecting.
It is yet another object of this invention to provide simple, low cost apparatus for conducting accurate and quantifiable geophysical aerial surveys and geophysical aerial prospecting.
It is yet another object of this invention to provide a method to carry out seismoelectric prospecting using natural seismic sources.
It is yet another object of this invention to provide simple, low cost apparatus for conducting accurate and quantifiable laboratory testing of geophysical samples.
It is yet another object of this invention to provide simple, low cost apparatus for locating a subterranean water table.
In general, the method of the invention uses the nature of propagation of seismic waves and the generation of seismoelectric signals by those waves to map subsurface features and permeability data. As a seismic wave travels through a water permeated subsurface matrix of earth materials, it generates seismoelectric signals. Previous researchers in the field did not realize that the data received showed that the seismoelectric signal was being generated at the same velocity and frequency as the seismic wave generating it. The seismoelectric signal then radiates (at the considerably greater velocity of a subterranean electromagnetic signal) away from that point of generation, while the seismic wave continues to generate new seismoelectric signals as it travels through the subsurface terrain. In the method of the invention, these seismoelectric signals are captured in the time and frequency domains by at least one pair of electrodes. The information they capture can then be analyzed using the method of the invention to tell the velocity and frequency of the original seismic wave. The propagation of seismic waves is relatively well understood, and the velocity and frequency data from that wave can be used to determine the subsurface topology, using methods well known in the art.
Compared to older, traditional, geophone surveying, the method is cheaper, more convenient, uses less complex A machinery, and, since the seismoelectric signals follow Ohm""s law, is very sensitive, not having a minimum threshold value for detection.